added more stubs to BASIC

c64/basic/a7ae.interop

@@ -1,3 +1,9 @@
-This routine is mentionned in the cartridge writing tutorial here: https://codebase64.org/doku.php?id=base:assembling_your_own_cart_rom_image
+This routine is used to launch other software. It is used by:
+
+- Exomizer (see https://github.com/bitshifters/exomizer/blob/master/exomizer/exomizer115.txt)
+- FileBrowser64 (FB64) - found by debugging
+
+and probably others. It is also mentionned in the cartridge writing tutorial here:
+
+- https://codebase64.org/doku.php?id=base:assembling_your_own_cart_rom_image
 
-We have to implement (or at least provide stub), otherwise cartridges will crash.